At present, the access networks of most telecommunications operators use cable, access technologies such as ADSL (Asymmetrical Digital Subscriber Line). Optical technologies are currently very little used access networks because the infrastructure cost as generated in particular by installing optical fibers between central offices and subscribers, is still prohibitive.
However, the use of optical technologies in access networks based on passive optical network architectures represents a significant step forward in terms of capacity, impossible to achieve with cable access technologies, but nevertheless inevitable given the increasing bit rates of services aimed at the subscriber.
Generally speaking, access networks in the form of passive optical networks can be of two types, standard passive optical networks and wavelength division multiplex (WDM) passive optical networks.
Standard passive optical networks use time division multiple access (TDMA) and require only one transmitter at the central office. They are based on using 1×N optical couplers (where N is a number of clients or subscribers) to distribute all the time division multiplexed data stream to the N clients. The information carried by a signal transmitted by the central office is then transmitted to all subscribers, and dedicated terminals on each subscriber premises extract the information intended for that subscriber. Thus data transmitted by the central office on a single wavelength is time division demultiplexed in each of the client terminals disposed on the subscriber premises.
However, the client terminal is complex and the attenuation of the signal by a 1×N coupler is non-negligible. Moreover, the fact that the information is extracted in each client terminal represents a security issue.
WDM passive optical networks use wavelength division of resources. In other words, each client is allocated a specific wavelength by the central office. Each wavelength is filtered in an optical demultiplexer and transmitted to the corresponding subscriber. That type of network therefore requires the use of a multiplex comprising a number of wavelengths equal to the number of subscribers and a demultiplexer.
A WDM passive optical network therefore has the advantages over a standard passive optical network of simplicity, each wavelength being assigned to a specific subscriber, and of performance, an optical demultiplexer causing significantly less attenuation than a 1×N coupler.
In contrast, because it uses more wavelengths and a routing component (an optical demultiplexer) that is more costly than a simple 1×N coupler, it is more costly.
There is also known a central office including a tunable laser that can emit a number of different wavelengths. It therefore transmits to clients one after the other by tuning the wavelength it transmits. However, the tunable laser must operate at a bit rate N times greater than that allocated to clients, and a switching time of 50 ns must be added, in the best case scenario, which is far from negligible with bit rate communications.